The present invention relates to self-clinching fastener and the method of assembly thereof. More particularly, the present invention concerns an improved form of fastener which will enable and insure obtainment of a predicted, predetermined locking action with both thick and relatively thin sheet material.
Self-clinching fasteners are well-known in the art and are of numerous designs, employing various clinching or staking methods. Primarily, these prior art fasteners are of the type which are assembled in a pre-punched hole in a sheet material, and then the head portion thereof is seated and embedded in the upper surface of the sheet material to cause displacement of a portion of said material radially inwardly of the aperture causing said material to co-flow into an annular locking groove formed in the fastener. The mechanical interlock thus created provides the means for maintaining the fastener in engagement with the sheet material.
These prior art fasteners and methods of assembly are subject to certain disadvantages. First of all, as the aperture in the sheet material is prepunched, the dimensional tolerances which may be encountered will result in a wide variance in the relative sizes of the fastener and the aperture. As such, in some instances the fastener makes fit snugly in the sheet material aperture prior to clinching, while in other instances an extremely loose fit will result. It can be appreciated that upon driving or embedding of the fastener head into the sheet material only a given amount of metal will be displaced and moved radially inwardly. Therefore, where relatively loose fits are encountered, there is often insufficient metal forced into the locking groove, resulting in a mechanical connection, that is, a subject of failure in use. As will become apparent from the hereinafter description, the present invention will provide means whereby a mechanical connection of controlled, predicted strength is obtained, regardless of the thickness of the sheet material.
The degree or strength of the mechanical connection between the fastener and the sheet material is dependent upon the shear area achieved by the mechanical interlock. That is to say, the shear area is defined as the cross-sectional area of the material displaced into the locking groove, taken in the direction of shearing, in this regard axially. Keeping in mind the above-discussed problem of tolerances, where incomplete filling of the locking groove results, the mechanical strength of the connection is not of the desired level. Also, it can be appreciated that it is desirable to make the width of the locking groove as great as possible in order to maximize the shear area. With thin materials, and prior art fasteners and methods of assembly, it was not possible to employ relatively wide grooves and thus the shear area of the mechanical interlock attained was inherently weak.
More specifically, with the prior art fastener designs employed with relatively thin material, the mechanical engagement between the fastener and the material was often insufficient to prevent backing out of the fastener during use. This was occasioned, due to the necessity of employing a groove having a width less than the sheet material thickness and the resulting, inherent low shear area thus obtained.
With the self-clinching fastener of the present invention, the sheet material aperture is reworked and sized to a desired, predicted configuration which not only assures close overlying engagement of the aperture walls with the fastener locking groove prior to clinching, but also can be utilized to increase greatly the amount of material available for displacement into the locking groove. This enables employment of a fastener with a relatively wide locking groove, and obtaining of the desired mechanical locking strength on both relatively thick and thin material.
The primary embodiment of the present invention achieves the above by employing an extruding nose portion on the fastener shank, which nose portion has a generally tapered configuration. The extruding portion has an external dimension which is greater than that of the initial aperture formed in the sheet material. Accordingly, as the fastener is moved inwardly of the aperture, the sheet material adjacent the aperture will be engaged by the extruding portion and will be cold worked as said extruding portion passes through said aperture.
Two important results are achieved with the abovediscussed structure and method of initial engagement. First, assuming use of relatively thick material, the difference in size between the initial aperture formed in the sheet material and the maximum dimension of the extruding portion will be slight, but sufficient to insure a reworking or resizing of the aperture. Once the extruding portion has passed through the aperture, the aperture will be resized with the aperture walls being in close sliding contact with the fastener shank overlying the locking groove. Thus, upon embedding of the head portion in the upper surface of the sheet material, substantially all of the material displaced inwardly will flow into the locking groove, rather than being utilized to overcome dimensional tolerances. Further, the volume of material displaced can be predicted and the size of said head and the volume of the annular locking groove adjusted to achieve the predetermined desired mechanical interlock depending of course upon the intended use and forces to which the connector will be subjected.
Considering now use with relatively thin material an additional advantage is achieved over and above that discussed in the preceding paragraph. As mentioned previously, with thin material and the prior art methods the width of the locking groove was limited. With the present invention, the initial size of the aperture and the relative cross-sectional dimension of the extruding portion are selected such that a considerable amount of sheet material will be forced inwardly as the extruding section passes through the sheet metal aperture. This action will produce a rimmed hole with a relatively long aperture wall, the length of said aperture wall being greater than the initial thickness of the sheet material. There is thus produced a resized aperture of increased effective length with a considerable amount of material disposed about the aperture wall. In addition, as was discussed in the preceding paragraph, the aperture walls will be in close, overlying engagement with the fastener shank. Accordingly, upon embedding of the head of the fastener in the upper surface of the sheet material, not only will the major portion of the displaced metal flow into the locking groove, but there is now afforded sufficient material to fill a relatively wide locking groove, a condition which could not be obtained without the resizing of the sheet metal aperture.
In addition to the above, the present invention also contemplates the employment of locking protuberances on the head which have a tapered undersurface, such that upon embedding of said protuberances in the sheet material, said tapered undersurface will assure radially inward movement of the displaced material, while precluding relative rotation.